Fix two null dereference bugs in SkSL visitor

Fuzzer found one of these, and I spotted the other during visual
inspection and manual testing.

Bug: oss-fuzz:25109
Change-Id: I8273231d9f76fe55459d4b742225627dcc524e6f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/312476
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed